Although lead has traditionally been used in numerous industrial applications, current regulations have mandated the elimination and/or phase out of lead in most commercial products. These mandates have stimulated new product development based upon lead-free technologies.
Soldering applications, particularly in electronics and vehicle manufacturing, have been heavily impacted by the ban on lead. Numerous alternatives to traditional lead-based solders have been developed (>300), the Sn/Ag/Cu (SAC) system being among the most widely used, but many have exhibited drawbacks that can make them unsuitable for use in certain applications. For example, SAC solder can be unsuitable for extreme environments such as those found in automotive, military, and space vehicles, where long life and reliability are of significant importance. SAC solder has a significantly higher eutectic melting point (m.p. of ˜217° C.) than does traditional Sn/Pb solder (m.p. of 183° C. for 63/37 Sn/Pb or 188° C. for 60/40 Sn/Pb), thus limiting its use to substrates that are capable of withstanding its relatively high working temperatures for effective processing (approximately 240° C.-270° C.). Even higher temperatures of approximately 260° C.-300° C. are more typically needed for rework of SAC solder to take place due to formation of high melting phases such as Ag3Sn and Cu6Sn5, further increasing the thermal demands of substrates upon which SAC is disposed. The need for high performance, thermally stable substrates for use in conjunction with SAC can significantly impact the cost of consumer products relative to those in which lower quality substrates can be used. In addition, silver is a relatively expensive component of the SAC system, and there is presently insufficient worldwide silver production capacity (22,000 tons/year) to allow total replacement of lead-based solders to take place with this system (90,000 tons/year). Still another limitation of SAC solder is that its high tin content makes it prone to tin whisker formation, which can increase the risk of electrical shorting.
Several compositions containing nanoparticles have also been proposed as replacements for traditional lead-based solders. Metal nanoparticles, particularly those that are about 20 nm or less in size, can exhibit a significant melting point depression over that of the corresponding bulk metal, thereby allowing the nanoparticles to be liquefied at temperatures comparable to those of traditional lead-based and lead-free solder materials. Copper nanoparticles, in particular, have been extensively studied as an alternative solder material. However, copper nanoparticle systems do not allow for easy rework to take place when replacement of failed components becomes necessary. Moreover, scalable processes for reliably producing bulk quantities of metal nanoparticles in a targeted size range are not yet well developed.
In view of the foregoing, nanoparticle compositions and scalable manufacturing processes thereof that address issues associated with current lead-free solder materials would be of substantial benefit in the art. The present invention satisfies the foregoing need and provides related advantages as well.